In a process for manufacturing semiconductor products, such as semiconductor integrated circuits, liquid crystal panels, solar batteries and panels thereof, and magnetic discs, a system is widely used in which plasma is generated under an inert gas atmosphere and a semiconductor product is subjected to various treatments using the plasma. As an inert gas forming the inert gas atmosphere in such a plasma treatment, rare gases, such as krypton and xenon, have been recently used. Since krypton and xenon are quite expensive, these gases contained in an exhaust gas discharged from the above-mentioned gas using facility are usually recovered and purified until the impurity concentration is reduced to 100 ppm or less. Also, other than the rare gases used in the above-mentioned plasma treatment, it is desired that fluoride gases used for cleaning and/or etching, such as NF3, CF4, C2F6, C3F8, and C4F8, a semiconductor material processing gas, such as arsine and phosphine, and heavy hydrogen gas for annealing gas are recovered, without being directly discharged outside, and purified to be reutilized.
The above-mentioned rare gases and the semiconductor material processing gases (hereinafter referred to as effective component gases) are discharged from a gas using facility generally in a state mixed with nitrogen gas. Other than nitrogen, moisture, carbon monoxide, carbon dioxide, hydrogen, hydrocarbons, metal hydride gases, halogenated hydrocarbon gases, etc., may also be contained in exhaust gas in minute amounts as impurities or reaction byproducts which may be generated in association with processes carried out in the gas using facilities and so forth.
As methods for separating a gas component contained in a mixed gas, a low temperature liquefaction separation method, a pressure swing adsorption (PSA) method, temperature swing adsorption (TSA) method, a membrane separation method, and so forth are known. In these methods, however, since a system is designed in accordance with a predetermined flow rate and composition of a mixed gas, a predetermined flow rate and degree of purity of a targeted component gas to be recovered, and so forth, it is difficult to efficiently carry out the separation of a targeted component gas.
In the above-mentioned semiconductor product manufacturing process in which a plasma treatment is carried out, since an atmospheric gas, normally nitrogen gas, which maintains a clean atmosphere in a treatment chamber, is introduced into the treatment chamber, most of the gas discharged from the treatment chamber consists of the nitrogen gas.
Then, the gas introduced into the chamber is changed from the nitrogen gas to a plasma treatment inert gas, for example, a rare gas, such as krypton, so that the rare gas is introduced into the treatment chamber prior to carrying out the plasma treatment. After the rare gas atmosphere is generated in the treatment chamber, the plasma treatment is performed, and the atmosphere in the treatment chamber is returned to the nitrogen atmosphere by switching the gas introduced into the treatment chamber to the nitrogen gas after the plasma treatment. The components contained in the exhaust gas change from a nearly all-nitrogen gas state to a nearly all-rare gas state due to a gradual increase of a concentration of the rare gas, and after the plasma treatment, the concentration of the rare gas is gradually decreased to return to the nearly all-nitrogen gas state. Also, since a vacuum pump used for drawing exhaust gas from the treatment chamber is generally one in which nitrogen gas is passed through the bearings, a part of the nitrogen gas is mixed with the exhaust gas.
Accordingly, in the above-mentioned semiconductor product manufacturing process, most of the components of the gas discharged during carrying in and out of a substrate to the treatment chamber and a standby state of the treatment chamber is nitrogen gas whereas the gas discharged during the plasma treatment contains nitrogen and rare gases. That is, the concentration of rare gas in exhaust gas changes from time to time. Also, since the flow rate of nitrogen gas and rare gas which are introduced into the treatment chamber are not always the same, the flow rate of exhaust gas changes accordingly. Note that the pressure of each gas when it is discharged is usually at atmospheric pressure.
As described above, if a conventional PSA system is used to separate and purify a target gas, for example, effective component gas, such as a rare gas, from an exhaust gas whose flow rate and concentration change from time to time, problems such as significant lowering in purity of collected rare gas due to unstable operational state and uneven recovery rate of a rare gas may be caused since the amount of exhaust gas introduced and the amount of product gas collected vary.
Also, conventionally, a replenishing rare gas is generally introduced at a preceding step in the gas using facility in order to replenish rare gas which will be discharged and not recovered. In such a case, however, it is required to store a rare gas for replenishment in a storage container at a pressure of 0.4 MPa or more since it is necessary to replenish rare gas at a pressure of 0.4 MPa or more. Accordingly, there is a problem in that a large amount of rare gas is initially required, and hence, the amount of rare gas used will increase, increasing the effective gas cost.